The present invention relates to gypsum boards and, more particularly, to gypsum boards having paper and/or fibrous mat facers adhered to a set gypsum core.
Panels of gypsum wallboard having a core of set gypsum sandwiched between two sheets of facing paper have long been used as structural members in the fabrication of buildings. Such panels are typically used to form the partitions or walls of rooms, elevator shafts, stairwells, ceilings and the like. Paper facing provides a smooth surface that is especially desirable for painting or wall papering interior walls. Although paper is a relatively inexpensive facing material and is easily used in the process of manufacturing wallboard, it has certain disadvantages, particularly with regard to moisture-resistance. As an alternative to paper facing, a fibrous mat (such as glass fiber) has been used as a facing material for gypsum wallboard. One example of such a wallboard is described in U.S. Pat. No. 3,993,822. Fibrous matting provides improved water resistance and often provides significant improvements in strength and other structural attributes. More recently, fibrous mats having various types of coatings also have found acceptance for use in applications requiring moisture resistance. See, for example, U.S. Pat. No. 5,552,187 and U.S. application Ser. No. 09/837,226, now Published Application No. U.S. 2002/0155282 A1.
Gypsum board is typically manufactured by a method which includes dispersing a gypsum slurry onto a moving sheet of facer. The facer typically is supported by equipment such as forming tables, support belts, carrier rolls and/or the like. A second sheet of facer is then fed from a roll onto the top of the slurry, thereby sandwiching the slurry between two moving facers. Forming or shaping equipment is utilized to compress the slurry to the desired thickness. The gypsum slurry is allowed to at least partially set and then sequential lengths of board are cut and further processed by exposure to heat, which accelerates the drying of the board by increasing the rate of evaporation of excess water from the gypsum slurry.
The composition from which the set gypsum core of the structural panel is made can include a variety of additives, such as set accelerators, set retardants, foaming agents, reinforcing fibers, and dispersing agents. In addition, a viscosity control agent may be added to adjust the viscosity of the slurry.
Examples of viscosity control agents are described in U.S. Pat. No. 4,647,496. Other typical additives include water-resistant additives and fire-resistant additives. A variety of additives for improving water-resistant properties of a gypsum core is described, for example, in U.S. Pat. No. 5,342,680, including a mixture of polyvinyl alcohol and a wax-asphalt emulsion.
To reduce the weight (density) of the core, it has been common practice to introduce small bubbles into the gypsum to produce a foamed gypsum core.
Foaming agents or soaps, typically long-chained alkyl sulfonates, are conventionally added for this purpose. One adverse consequence of the normal addition of soaps into gypsum slurry is a reduction in the strength of the bond between the cured gypsum core and the paper facers. To counteract this effect, a starch binder normally is added to the gypsum slurry.
U.S. Pat. No. 4,518,652 to Willoughby describes preparing a set cementitious product, such as paper-faced gypsum wallboard, in which polyvinyl alcohol is used as a foaming agent. During manufacture, a foamed solution of polyvinyl alcohol is introduced into the aqueous calcined gypsum slurry. The process is said to avoid the need for conventional detergents or soaps as foaming agents, while providing good adhesion between the paper cover sheets and the gypsum core. However, in the examples starch is added in the gypsum core and apparently is needed to obtain adequate adhesion to the paper facer sheets.
It would be desirable to develop a lightweight gypsum board that is free or substantially free of starch and yet has adequate adhesion properties between the core and paper and/or fibrous mat facers, especially coated fibrous mats.
One aspect of the present invention is directed to a process of preparing a gypsum board having adequate adhesion between a set gypsum core and at least one adjacent facer sheet. A relatively thin coating of an aqueous slurry of calcined gypsum containing an adhesion promoting effective amount of polyvinyl alcohol is applied to a first side of a first facer sheet. A relatively thin coating of aqueous gypsum slurry containing an adhesion promoting effective amount of polyvinyl alcohol optionally is applied to a first side of a second facer sheet. An aqueous slurry of calcined gypsum is applied to the first or second facer sheet to form the core of the board. The first and second facer sheets are pressed together to form a wet board, and the wet board is dried to form a gypsum board.
Another aspect of the present invention is directed to a gypsum board having adequate adhesion between a set gypsum core and adjacent facer sheets without the need for starch or other conventional binders. The gypsum board comprises a set gypsum core containing an adhesion promoting effective amount of polyvinyl alcohol, a first facer sheet adhered to a first surface of the set gypsum core, and a second facer sheet adhered to a second surface of the set gypsum core. The set gypsum core is free or substantially free of starch.
In one preferred embodiment, the polyvinyl alcohol is concentrated near that portion of the set gypsum core adjacent to the facer sheet(s) and is not present in the center of the set gypsum core.
The present invention provides an economical, lightweight gypsum board having good adhesion properties between the gypsum core and the adjacent facer sheets without the need for starch or other conventional binders, thereby overcoming certain drawbacks associated with presently available gypsum board technologies.